Glass material

ABSTRACT

A glass including SiO 2 , Na 2 O, MgO, Al 2 O 3 , and cobalt oxide, wherein the cobalt oxide is 4.5-85 wt % as an oxide of CoO or 4.9-91 wt % as an oxide of Co 3 O 4 .

[0001] This is a Continuation of application Ser. No. 09/722,503, filedNov. 28, 2000, which is a Continuation of U.S. application Ser. No.09/432,782, filed Nov. 3, 1999, now U.S. Pat. No. 6,177,169, issued Jan.23, 2001, which is a continuation of U.S. application Ser. No.09/090,382, filed Jun. 4, 1998, now U.S. Pat. No. 5,985,401, issued Nov.16, 1999, the subject matter of which is incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to an information recording medium,and more particularly, to an optical information recording medium, whichis capable of reading out or recording with a high recording density,and which has a high reliability in repeating recording and regenerationoperations.

[0003] Conventionally, compact disks (CD), laser disks (LD), and thelike are used widely as optical information recording media. Currently,a DVD, which has seven times the recording density of a CD, has comeinto practical use. The DVD is being developed as an erasablerecording-regenerating medium in addition to a read only medium(DVD-ROM), wherein information is directly written onto the substrate.Furthermore, the practical use of a DVD as a RAM for a computerpresently is under investigation.

[0004] With the DVD, high density recording can be achieved by using alaser having a shorter wave length, such as 650 nm, than the laser usedfor a CD (wave length approximately 780 nm). However, in order to handlea large amount of information, such as computer graphics and the like,it is necessary to achieve a higher recording density, such as 1.5 to 2times that of the conventional high density recording. In order toachieve such a high recording density, a semiconductor laser of green toblue color having a shorter wave length (wave length 520-410 nm) thanever is under development.

[0005] As another means to achieve a higher recording density, a superresolution film can be employed. The super resolution film is a thinfilm formed at a lower plane of the recording medium, with which a highrecording density can be achieved by the fact that it is able todecrease the size of the beam spot of the incident light passing throughthe film.

[0006] One of the mechanisms of the super resolution effect is anabsorption-saturation phenomenon, which is a phenomenon utilizingnon-linear optical characteristics of the super resolution film suchthat the film allows light having a larger intensity than the amount ofits absorption-saturation to pass through the film and absorbs any lighthaving an intensity less than the amount of its absorption-saturation.The spatial intensity of a laser beam utilized in reading and writinghas a Gaussian distribution. Therefore, when the laser light beam passesthrough the super resolution film, the laser light in the lower endportion of the Gaussian distribution, where the intensity is low, isabsorbed by the film, and the laser light in the middle portion of theGaussian distribution, where the intensity is high, passes through thefilm. Accordingly, the diameter of the laser beam is reduced as itpasses through the super resolution film.

[0007] An organic thin film made of a material in the phthalocyaninegroup, as disclosed in JP-A-8-96412 (1996), chalcogenide, fine particlesof a compound semiconductor, and the like are known at the present asmaterials which may be used for the super resolution film describedabove. Additionally, trials to use some organic materials, such asthermochromic materials of the type disclosed in JP-A-6-162564 (1994),and photochromic materials of the type disclosed in JP-A-6-267078(1994), as the super resolution film have been carried out.

[0008] However, the above-mentioned materials have problems inreliability and productivity. That is, there has been a concern aboutgradual deterioration of the organic thin film after repeated recordingand regenerating operations, because the energy density of a laser beamis locally increased significantly during the recording and regeneratingoperations. Therefore, a sufficient guarantee period for the recordingand regenerating operations is scarcely obtained under a severecondition of use, wherein the recording and regenerating operations areperformed frequently, such as when the disk is used as a RAM and thelike for computers.

[0009] On the other hand, chalcogenide is chemically unstable, and so along guarantee period can not be obtained for this material, and thefine particles of a compound semiconductor provide difficulties duringthe production process.

SUMMARY OF THE INVENTION

[0010] One of the objects of the present invention is to provide anoptical recording medium having a super resolution film, which canguarantee repeated recording and regenerating operations for asufficiently long time, and which has a preferable productivity and ahigh resolution effect.

[0011] A first aspect of the present invention to solve the above issuesis an optical information recording medium comprising a substrate,whereon a recording layer for recording information is formed; and aglass thin film, formed onto the substrate, having a characteristicssuch that the intensity distributions of irradiated light onto the glassand transmitted light through the glass vary in a non-linear manner.

[0012] The substrate is desirably transparent to light, and forinstance, is made of inorganic materials, such as glass and the like,and organic materials, such as polycarbonate, polyethyleneterephthalate, and the like are also desirable. Here, the term glassrefers to amorphous solid oxides and general amorphous materialscontaining the above oxide as a main component.

[0013] Forming on a substrate includes both forming onto the surface ofa substrate directly and forming onto the surface of a substrateindirectly via another layer, for instance, a protection layer.

[0014] In accordance with the above composition, an informationrecording disk, which has a large capacity, and which experiences lessdeterioration after repeated reading out and writing, can be provided.

[0015] In the first aspect of the invention, the recording layer can beprovided with a pit pattern representing the recording information. Thepit pattern is a device by which the information is recorded inaccordance with the arrangement of pits provided onto the surface of thesubstrate. If this recording method is employed, the recordedinformation can not be rewritten. However, once a master die of thesubstrate having this recorded information is made, a large number ofsubstrates with the same information can be manufactured readily.Therefore, this recording method is used for recording movies, music,and computer programs.

[0016] The recording layer of the invention can also be a device forrecording information with optical energy. For recording informationwith optical energy, an information recording substrate using so-calledphase changing organic materials or inorganic materials, the crystallinestructure of which varies when irradiated by light, is used as therecording layer.

[0017] A second aspect of the present invention is an opticalinformation recording medium comprising at least a substrate, arecording layer for recording information formed on the substrate, and areflecting film for reflecting light formed on the recording layer,wherein the substrate is made of glass, the optical transmittance ofwhich increases in a non-linear manner corresponding to an increase inintensity of the irradiated light.

[0018] In accordance with the above composition, a reflection typeinformation recording disk, which has a large capacity and lessdeterioration after repeated reading out and writing, can be provided.

[0019] This second aspect of the invention provides an informationrecording substrate of a type, which reflects incident light with areflecting film provided at a lower portion of the recording film, andreads the information with reflected light.

[0020] The glass in the first or the second aspects of the inventiondesirably contains at least an element selected from transition metalelements and rare earth metal elements.

[0021] For the above transition metal elements and the rare earth metalelements, particularly, at least an element selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co, Ni, Nd, Ce, Pr, Sm, Eu, Tb, Ho, Er,and Tm is desirable.

[0022] When the transition metal element or the rare earth metal elementforms a glass film, the metal element is desirably contained in therange from 20% by weight to 90% by weight as an oxide to the totalweight of the glass. When the metal element forms a glass substrate, themetal element is desirably contained in the range from 0.1% by weight to29% by weight as an oxide to the total weight of the glass.

[0023] In the first aspect of the invention, the glass desirablycontains as oxide the following compounds: SiO₂: 6-80% by weight, R₂O :0-20% by weight (R=alkali metal element), B₂O₃ : 0-30% by weight, andCoO : 20-90% by weight.

[0024] In the second aspect of the invention, the glass desirablycontains cobalt oxide as CoO in the range of 0.1-29% by weight.

[0025] A third aspect of the present invention is an informationrecording medium comprising at least a substrate, whereon a recordinglayer for recording information is formed, and a super resolution layerformed on the substrate, the optical transmittance of which increases ina non-linear manner corresponding to an increase in the intensity of theirradiated light, wherein an output maintaining rate of the informationrecording medium after repeating the recording by 10⁴ times is at least90%.

[0026] The output maintaining rate is a value indicating how much of theintensity of the electrical signal is maintained after repeating therecording and regeneration by 10⁴ times, taking the intensity of theelectrical signal at the first regeneration of information afterperforming the first recording with irradiation of light as 100%. If thesuper resolution film is deteriorated by repeating the irradiation oflight, the spot size of the laser ray which reaches the recording layeris expanded, and, as a result, the electric output is decreased. Thatmeans that a super resolution film which can maintain the initial outputmaintaining rate as long as possible is desirable.

[0027] Furthermore, in accordance with a fourth aspect of the presentinvention, an information recording medium is provided, which comprisesa transparent substrate, and a recording layer for recording informationwhich is formed onto the substrate, wherein an output decrease inrecorded signal at a frequency of 8 MHz is less than −30 dB of theoutput at 1 kHz, and an output maintaining rate after repeating therecording by 10⁴ times is at least 90%.

[0028]FIG. 8 is a graph indicating a relationship between the recordingfrequency and the output for the information recording media with andwithout the super resolution film of the present invention. The mediumwith the super resolution film can record signals of higher frequencycomponents, because the spot size of the laser beam reaching therecording layer is decreased. The above composition indicates an indexwhich represents how high a frequency component can be recorded.

[0029] In accordance with a fifth aspect of the present invention, glasscomprising SiO₂: 6-80% by weight, R₂O: 0-20% by weight (R=an alkalimetal element), B₂O₃ : 0-30% by weight, CoO : 20-90% by weight, asequivalent oxide, respectively, is provided.

[0030] The above glass can be mounted not only on a photo disk, but alsoon various media, as a film having the super resolution effect. Forinstance, a display apparatus, which generates light when itsfluorescent body is irradiated with a laser ray so as to be excited, canproduce a high resolution display by mounting the grass film of thepresent invention onto a surface of the fluorescent body, because thespot size of the laser ray can be converged.

[0031] In accordance with a sixth aspect of the present invention, aglass thin film containing cobalt oxide in the range of 20-90% by weightas equivalent CoO is provided.

[0032] In the case of this glass film, the upper limit of the CoOcontent is restricted, because, if CoO is added excessively, the CoO isprecipitated, and causes devitrification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The above and further objects and novel feature of the presentinvention will more fully appear from the following detailed descriptionwhen the same is read in connection with the accompanying drawings. Itis to be expressly understood, however, that the drawings are forpurpose of illustration only and are not intended as a definition of thelimits of the invention.

[0034]FIG. 1 is a schematic cross section of a RAM disk according to thepresent invention;

[0035]FIG. 2 is a schematic cross section of a simulated sampleaccording to the present invention;

[0036]FIG. 3 is a graph indicating a dependency of transmittance on wavelength of the glass thin film according to the present invention;

[0037]FIG. 4 is a diagram showing an XPS of Co of a glass thin filmaccording to the present invention;

[0038]FIG. 5 is a graph indicating a relationship between transmittancefor light of 650 nm and CoO content;

[0039]FIG. 6 is a diagram showing a SIMS of a glass thin film formedonto a glass substrate having a target composition;

[0040]FIG. 7 is a schematic cross section of a ROM disk according to thepresent invention;

[0041]FIG. 8 is a graph indicating a reading out frequency dependence ofan output obtained from the ROM disk shown in FIG. 7;

[0042]FIG. 9 is a graph indicating a relationship between mark lengthand variation in output obtained from the RAM disk shown in FIG. 1;

[0043]FIG. 10 is a graph indicating a dependency of output on therepeating of operations on the RAM disk shown in FIG. 1;

[0044]FIG. 11 is a graph indicating a relationship between CoO contentand variation in reading out of the output obtained from the RAM diskshown in FIG. 1;

[0045]FIG. 12 is a schematic cross section of a RAM disk according tothe present invention;

[0046]FIG. 13 is a schematic cross section of a ROM disk according tothe present invention;

[0047]FIG. 14 is a graph indicating variations of laser beam diameterwhen the glass film of the present invention is formed and not formed;and

[0048]FIG. 15 is a block diagram of an apparatus using the photo disk ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] (Embodiment 1)

[0050] Details of the present invention will be explained hereinafterwith reference to various preferred embodiments.

[0051] The composition of a number of glass targets investigated in thedevelopment of the present invention is indicated in Table 1. TABLE 1Composition (% by weight) Target Film No. SiO₂ Na₂O CaO MgO Al₂O₃ Co₃O₄CoO (kind) Q¹) 1 70.4 13.6 7.8 4.0 1.3 2.9 — glass ◯ 2 69.0 13.3 7.6 3.91.3 2.9 — glass ◯ 3 51.8 10.0 5.7 2.9 1.0 28.6 — glass ◯ 4 45.3 8.8 5.02.6 0.9 37.4 sint²) ◯ 5 29.0 5.6 3.2 1.6 0.6 60.0 sint ◯ 6 14.5 2.8 1.60.8 0.3 80.0 sint ◯ 7 5.9 1.4 0.5 0.2 0.1 91.9 sint X 8 — — — — — 100sint X 9 51.8 10.0 5.7 2.9 1.0 — 28.6 glass ◯ 10 29.0 5.6 3.2 1.6 0.6 —60.0 sint ◯ 11 14.5 2.8 1.6 0.8 0.3 — 80.0 sint ◯ 12 5.9 1.4 0.5 0.2 0.1— 91.9 sint X 13 — — — — — — 100 sint X

[0052] In Table 1, the column indicating the film quality was providedwith O when a uniform film was obtained in view of transparency,uniformity, and the like, and with X when the obtained film was notuniform.

[0053] In the present embodiment, a soda-lime group glass was used as amother glass, and a cobalt oxide, which had a large absorption in thevicinity of 650 nm, was used as the transition metal. As raw materialsfor the cobalt oxide, CO₃O₄ and CoO were used.

[0054] The targets No. 1-No. 7 are composed of soda-lime glass andCO₃O₄. Among them, targets No. 1-No. 3 were targets in the form of aglass block, because they were vitrified. Targets No. 4 No. 7 were notvitrified, because the content of CO₃O₄ was too much to be vitrified.Therefore, a sintered body of a mixture of glass powder and CO₃O₄ wasprepared as a sintered target.

[0055] Target No. 8 is a comparative example of a sintered target madeof only CO₃O₄.

[0056] In targets No. 9-No. 13, CoO was used as the raw material forcobalt oxide. In these cases, target No. 9 was a glass target, becausetarget No. 9 had a Co content of 28.6% by weight and was vitrified.Because targets No. 10-No. 12 were not vitrified, a sintered target ofthe mother glass raw material and CoO was used.

[0057] Target No. 13 is a comparative example of a sintered target madeof only CoO.

[0058] The glass block for the target was obtained by the steps ofweighing a designated amount of powdered raw materials, charging thepowdered raw materials into a crucible made of platinum, heating thecrucible to approximately 1500° C. in an electric furnace to melt theraw materials, and pouring the molten glass into a graphite mold, whichwas pre-heated to approximately 400° C. After the raw materials weremolten completely, the molten material was cooled rapidly, a stressrelief was performed by reheating the material to approximately 600° C.and then cooling it gradually, followed by polishing the back side ofthe obtained glass block.

[0059] The sintered target was obtained by the steps of granulating adesignated amount of powdered raw materials, fabricating the powder intoa fabricated body in a die, and hot pressing the fabricated body at adesignated temperature after dewaxing. The temperature for heattreatment was 900° C. when the cobalt raw material was CO₃O₄, and was1200° C. when the cobalt raw material was CoO.

[0060] As a previous step the evaluation of the shape of the disk, aglass sample for a preliminary test in the shape of a thin film as shownin FIG. 2 was prepared, and fundamental material characteristics of theglass thin film were determined. In FIG. 2, the numeral 1 indicates asubstrate, and the numeral 2 represents the glass thin film. In thepresent investigation, a soda-lime glass 0.55 mm thick and 30 mm squarewas used as the substrate 1.

[0061] The structure of the prepared film was evaluated by a thin filmX-ray diffraction method. Then, it was found that all the prepared filmswere amorphous regardless of whether the target was glass or a sinteredbody, and that glass films were formed.

[0062]FIG. 3 indicates the dependence of the transmittance of the glassthin films formed using the targets shown in Table 1 on wave length. Thetransmittance was measured using monochromatic light obtained bytreating white light from a light source with a monochromator. Inaccordance with target No. 1, the peak indicating an absorption washardly observed around 300 nm, because of too small a content of CO₃O₄.In accordance with targets No. 2-No. 4, the peak indicating theabsorption by Co could be observed, even though it is small, in theregion of 500 nm-700 nm. No. 3 glass had a transmittance ofapproximately 85% at a wave length of 650 nm.

[0063] With the thin films of targets No. 5 and 6, the values of thetransmittance were sufficiently low. However, the transmittance wasdecreased in accordance with decreasing wave length, and it wasindicated that the decrease in the transmittance was caused byscattering. The glass of target No. 7 and the CO₃O₄ of target No. 8 hada sufficiently low transmittance. However, they were reduced in aspattering atmosphere, and a film having a metallic luster was obtained.Therefore, the transmittance was decreased by reflection.

[0064] On the other hand, in accordance with the glasses of targets No.9-No. 11, using a raw material of CoO, a peak indicating absorption byCo was observed in the vicinity of the region of 500 nm-700 nm. Thetransmittance was decreased in accordance with the increase in Cocontent. The thin film of target No. 11 containing 80% of Co had atransmittance of approximately 5% at wave length of 650 nm. The glass oftarget No. 12 containing CoO of 91.9%, and the film of target No. 13,which was 100% CoO, indicated the same results as target No. 8.

[0065] In order to investigate the difference in spectrum of thetransmittance curves in FIG. 3, the valence and oxide conditions of theCo were analyzed by XPS. The XPS spectra of Co in the thin films oftargets No. 3 and 5 are indicated in FIG. 4. In the spectrum of the thinfilm of target No. 3, a peak called a shake up peak exists around 786eV. It indicates the presence of a large amount of Co²⁺ . On thecontrary, the shake up peak can not be observed in the spectrum of thethin film of target No. 5. It indicates an oxide condition of CO₃O₄coexisting with Co³⁺ . Accordingly, scattering occurred, and the profileindicated in FIG. 3 was obtained.

[0066] The same investigation was performed with other thin films, andit was found that, if cobalt existed in the condition of Co²⁺, thespectrum included the peak of absorption typical for Co, such as intargets No. 2, 3, 10, and 11, and, if Co³⁺ existed, the spectrum becamea curve accompanied with scatter, such as in targets No. 5 and 6.

[0067]FIG. 5 indicates the relationship between the plottedtransmittance at a wave length of 650 nm versus the Co ion content in atarget based on the thin film transmittance curves of targets No. 2, 3,10, and 11. The transmittance was decreased in accordance withincreasing CoO content, and the transmittance became approximately 30%when the CoO content was 60%.

[0068] Then, in order to evaluate the Co content in the prepared glassthin films, a composition analysis of the film was performed with asecondary ion mass spectrometer (SIMS). A plate cut out from the glasshaving the same composition as the target was used as a substrate, and athin film having the same composition was spattered onto the substrate.The analysis was performed from a film forming direction to a depthdirection, so that the compositions of the film and the substrate couldbe evaluated continuously. In the present embodiment, the investigationwas performed using target No. 3 as the target composition.

[0069] The results of the analysis are indicated in FIG. 6. It was foundthat the Co content in the thin film was larger than that in thesubstrate. The Si content in the thin film was smaller then that in thesubstrate. However, the amounts of change were small, and a largedeflection in the composition could not be expected. Therefore, the filmcomposition can be regarded approximately as being the same as that ofthe target composition.

[0070] In accordance with the above investigation, the Co oxide contentin the glass thin film is desirably in the range of from 4.5% by weightto 85% by weight as an oxide of CoO, and of from 4.9% by weight to 91%by weight as an oxide of CO₃O₄. If CoO is less than 4.5% by weight, itis difficult to obtain a sufficient absorption of light. If CoO exceeds85% by weight, the film bears a metallic luster, and the transmittanceis decreased.

[0071] (Embodiment 2)

[0072] Then, the super resolution effect was evaluated by manufacturingROM disks, whereon the glass film of the present invention was formed.

[0073]FIG. 15 is a block diagram of an example of the optical recordingapparatus used with the optical disk of the present invention. Using theoptical recording apparatus having the above composition, theperformance of the ROM disk of the present invention was evaluated. Thesame apparatus was used on other embodiments.

[0074]FIG. 7 indicates a schematic cross section of the manufactured ROMdisk. In FIG. 7, the disk includes a polycarbonate substrate 1, a glassthin film 2, a SiO₂ protective film 5, and a Al reflector 4, and pits 6represent stored information.

[0075] The ROM disk was manufactured by the following steps First, a pitpastern representing information was formed onto a photoresist by alaser. The pit pattern was duplicated onto a Ni die, and substrates wereformed by injection molding polycarbonate into the Ni die. A glass film160 nm thick was formed onto the substrate by spattering, and after aSiO₂ protective film of 140 nm thick was formed thereon, an aluminumreflecting film 100 nm thick was formed. In the present embodiment, thetarget No. 11 film was formed as the glass thin film. As a comparativeexample, a ROM disk without forming the glass film also wasmanufactured.

[0076] The frequency dependency of the regenerating output intensity ofthe manufactured ROM disk was analyzed with a spectrum analyzer. Theresults are indicated in FIG. 8. The regenerated laser power is 4 mW. Itwas revealed that, in a case when the glass thin film of target No. 11was formed, the output level was high until frequency components becamehigher than a case when the glass thin film was not formed. Since thehigh frequency components of a signal are written with a finer pitpattern on the ROM disk, the above result indicated that the output wasregenerated by reading out a finer pit pattern when the glass film wasformed. Therefore, it was found that, when the glass film was formed,the super resolution effect had been obtained.

[0077] The same investigations as the above were performed on otherglass films in Table 1, and the same super resolution effect wasconfirmed on the glass films of targets No. 3-6, and No. 9-11.

[0078] Then, a RAM disk, wherein the glass thin films investigated abovewere formed on the substrate, was manufactured, and its characteristicswere evaluated. A schematic cross section of the RAM manufactured inaccordance with the present invention is indicated in FIG. 1. In FIG. 1,the disk includes a polycarbonate substrate 1, a glass super resolutionfilm 2, a recording film 3, a reflecting film 4, and protective films 5,5′. In accordance with the present invention, a circular plate 0.6 mmthick and 120 mm in diameter was used as the polycarbonate substrate 1.A glass film 300 nm thick was formed thereon by a spattering method toform the super resolution film 2. After forming a ZnS SiO₂ protectivefilm 80 nm thick thereon, a Ge-Sb-Te group phase changing filmrepresenting the recording film was formed approximately 20 nm thickthereon by the same spattering method. Then, after forming a protectivefilm approximately 90 nm thick, an AlTi reflecting film 200 nm thick wasformed thereon.

[0079] The glass thin film was formed by the following steps. That is, aglass block or a sintered body 5 mm in thickness and 120 mm in diameterwas manufactured as a target, and a backing plate made of copper wasadhered onto the back side of the target with an organic adhesive agentfor vacuum. Spattering was performed using Argon gas. The power was 200mW. The film was formed uniformly by rotating the substrate during thespattering. In the present embodiment, the sample target No. 11 was usedas the glass film. As a comparative example, a RAM disk, whereon thegrass film was not formed, was manufactured.

[0080]FIG. 9 indicates a relationship of recording mark length versusregenerating output intensity of the RAM disk, whereon recording marksof the same shape were formed with an equal interval. The laser powerfor reading out was 2 mW. In accordance with FIG. 9, it was revealedthat the present embodiment, whereon the glass film of target No. 11 wasformed, had higher regenerating outputs than the comparative example,which did not have the glass film, in the shorter mark length region.Therefore, it was revealed that regeneration was possible to the shortermark length when the glass film is formed. Accordingly, the superresolution effect could tee confirmed with the RAM disk.

[0081] The same results as the case of the RAM disk were obtained whenall the glass films shown in Table 1 were investigated.

[0082] Then, a spatial intensity distribution of the reflecting light inthe cases when the above super resolution effect was obtained wereinvestigated. FIG. 14 indicates schematically the intensity distributionof laser light in the laser beam forwarding direction both in the casewhen the glass film was formed and the super resolution effect wasobtained, and in the case when the glass film was not formed. Inaccordance with FIG. 14, it was revealed that the spatial intensitydistribution was approximately a Gaussian distribution in the case whenthe grass film was not formed, but the distribution of the beam wasdeflected toward the laser beam forwarding direction when the glass filmwas formed.

[0083] Simultaneously, it was revealed that the beam diameter Q' at thebeam intensity necessary for reading out became smaller in comparisonwith the case when the glass film was not formed.

[0084] In accordance with the above results, it was revealed that theintensity and the intensity distribution of the reading out light couldbe varied by using the grass film such as provided in the presentembodiment. Furthermore, it was revealed that the super resolutioneffect could be obtained in the above case.

[0085] (Embodiment 3)

[0086] Next, deterioration of the film by repeated regeneration wasevaluated. The evaluation was performed by repeatedly irradiating themanufactured RAM disk with a regeneration signal light and detecting theregenerated output. The pit pitch was 0.3 μm. The glass thin film oftarget No. 11 was used. As a comparative example, a phthalocyanine grouporganic thin film was selected, and the same evaluation was performed.

[0087]FIG. 10 indicates a relationship between the output versus therepeated number of operations. In accordance with FIG. 10, it wasrevealed that the output of the disk formed with the organic group thinfilm was decreased gradually over the repeated regenerationsapproximately 10,000 times. On the contrary, the output of the diskformed with the glass thin film of the present invention was hardlydecreased by repeating the regeneration over 10,000 times. As explainedabove, it was revealed that the optical disk of the present inventionmaintained the super resolution effect even after repeated regeneration.

[0088] The high stability against repeated regeneration could beobtained when the glass thin film, with which the super resolutioneffect was obtained in the above embodiment 2, among other glass filmsin Table 1, was used as the glass thin film.

[0089] (Embodiment 4)

[0090] Then, the composition of the glass thin film was investigated.First, paying attention to the content of cobalt oxides in the glassfilm, a relationship between the cobalt content and the output power wasinvestigated by manufacturing the same RAM disk as the disk in theembodiment 2. The mark length was 0.3 μm. The laser power was 2 mw. FIG.11 indicates a relationship between the cobalt consent and the output.The output power was increased in accordance with an increasing cobaltcontent, and it was found that a high output could be obtained even witha small mark length. In other words, it was found that the superresolution effect could be increased in accordance with an increasingcobalt content. Furthermore, it was found that the output exceeded 5 dBwhen the cobalt content was equal to or more than 20%, in which case itwas possible to beat the output as a signal. However, when the cobaltcontent was less than 20%, the output was less than 5 dB, and it wasimpossible to beat the output as a signal.

[0091] The ROM disk shown in FIG. 7 was manufactured, and the output tothe high frequency component was evaluated by a spectrum analyzer. Then,it was revealed that the high frequency component could be read out whenthe cobalt content was equal to or more than 20%, but when the cobaltcontent was less than 20%, any significant effect of adding cobalt couldnot be observed.

[0092] In accordance with the above investigation, the cobalt contentdesirably should be equal to or more than 20% by weight in any ease ofboth a ROM and a RAM. In accordance with the investigation in theembodiment 1, the cobalt content desirably should be equal to or lessthan 91% by weight.

[0093] Furthermore, chemical elements to be contained in the glass filmwere investigated. The mother glass was soda lime glass. The glasscontaining an oxide of at least one element selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu among transitionmetallic elements, and Nd, Ce, Pr, Sm, Eu, Tb, Ho, Er, and Tm among rareearth elements had an absorbing spectrum typical of the respectiveelement, and the same super resolution effect as the embodiment 2 couldbe obtained by using a laser beam having a wavelength band capable ofabsorption.

[0094] In accordance with the above results, an optical disk having thesuper resolution effect could be obtained by using the glass thin filmcontaining at least one element selected from the group consisting ofTi, V, Cr, Mn, Fe, Co, Ni, Cu, Nd, Ce, Pr, Sm, Eu, Tb, Ho, Er, and Tmamong transition metallic elements and rare earth elements.

[0095] Next, the composition of the mother glass was investigated. Inthe above embodiments, soda lime glass was used as the mother glass.However, the same effect could be obtained by using borosilicate glasscontaining boron. However, when the content of SiO₂ was less than 6% byweight, the stability as glass was low, and crystallization and the likecould occur when containing an oxide of the transition element or therare earth element. When the content of SiO₂ exceeded 80% by weight, theabove oxide could be hardly included into the glass structure, and itwas difficult to obtain a stable glass. In accordance with the aboveresults, the content of SiO₂ desirably should be in the range from 6% byweight to 80% by weight.

[0096] When the content of alkaline oxide in the glass exceeded 20% byweight, the durability of the glass decreased, and, obtaining a stableglass was difficult. Accordingly, the content of the alkaline oxidedesirably should be equal to or less than 20% by weight.

[0097] Furthermore, when the consent of boron oxide in the glassexceeded 30% by weight, the oxide of the transition metal or the rareearth element was hardly included in the glass structure, and obtaininga stable glass was difficult. Therefore, the content of boron oxidedesirably should be equal to or less than 30% by weight.

[0098] In addition to the above indispensable components, an oxide ofalkaline earth elements, alumina, zirconia, and the like are desirablycontained in the glass as a glass stabilizing agent.

[0099] (Embodiment 5)

[0100] Next, the super resolution effect was investigated bymanufacturing glass substrates containing a transition metallic element.FIG. 12 indicates schematically a cross section of a manufactured RAMdisk. In FIG. 12, the disk includes a glass substrate 12, a recordingfilm 3, a reflecting film 4, and protective films 5, 5′. The thicknessof the substrate was 0.6 mm, and a track was formed onto the surface ofthe substrate by reactive ion etching using a photoresist as a mask.FIG. 13 indicates schematically the cross section of the ROM diskmanufactured using the same substrate. In FIG. 13, the disk includes aglass substrate 12, a reflecting film 4, and a recording mark 6representing written information. In the present embodiment, soda limeglass was used as the mother glass, and CoO was contained therein as thetransition metallic oxide. The super resolution effect was investigatedusing the same evaluating method as the embodiment 2 by varying theconsent of CoO in the range of 0.01-30% by weight.

[0101] The composition of the manufactured glass substrate, theevaluated results in vitrification and the super resolution areindicated in Table 2. In the evaluated results of the vitrification, thecase when glass was formed without causing crystallization was indicatedwith O, and the case when crystallization or devitrification was causedwas indicated with X. The evaluated results of the super resolutioneffect were indicated by the mark length of 0.3 μm and the output at thespace length. The reading out laser wavelength was 650 nm. TABLE 2S.R.E. Composition (% by weight) Vitri- (output/ No. SiO₂ Na₂O CaO MgOAl₂O₃ CoO fication db)^(*1) 14 72.5 14.0 8.0 4.1 1.4 0.01 ◯ 5 15 72.514.0 8.0 4.1 1.4 0.05 ◯ 5 16 72.5 14.0 8.0 4.1 1.4 0.10 ◯ 10 17 71.813.9 7.9 4.0 1.4 1.00 ◯ 16 18 65.9 12.7 7.3 3.7 1.3 9.1 ◯ 33 19 51.810.0 5.7 2.9 1.0 28.6 ◯ 41 20 50.7 9.8 5.6 2.9 1.0 30.0 X — 21 66 9B₂O₃9 1 5 10.0 ◯ 33 22 49 9 26 1 5 10 ◯ 35 23 43 9 32 1 5 10 X —

[0102] In accordance with the specimens No. 14-19, and 21-23, nocrystallization nor devitrification were observed, and stable glassescould be manufactured. The glass of specimen No. 20 caused a phaseseparation after pouring, and so a stable glass could not be obtained.

[0103] In view of the above result, the content of Coo in the glassdesirably should be equal to or less than 29%.

[0104] The glasses of specimen No. 21-23 were the same as the glasses ofspecimen No. 14-19, except for replacing their CaO with B₂O₃.

[0105] Regarding the regenerating output, outputs not less than 10 dBcould be obtained when the cobalt content was equal to or more than0.10% by weight, and it was possible to read them out as signals. On thecontrary, when the cobalt content was equal to or less than 0.05% byweight, the outputs were as small as less than 5 dB, and it wasimpossible to read them out.

[0106] In accordance with the above results, the content of cobaltdesirably should be in the range of 0.10-29% by weight. The aboveeffects were similar with the glasses which contained B₂O₃ instead ofCaO.

[0107] In accordance with the present invention, an informationrecording disk having a large capacity, and a small deteriorationagainst repeated reading out and writing in operations, can be provided.The present invention can provide an optical disk having a largecapacity when manufacturing it with a conventional optical diskmanufacturing process.

What is claimed is:
 1. A glass comprising SiO₂ and cobalt oxide, whereinsaid cobalt oxide is 4.5-85 wt % as an oxide of CoO or 4.9-91 wt % as anoxide of Co₃O₄.